Re: [PATCH v5 19/20] x86/efistub: Perform SNP feature test while running in the firmware

[Tom Lendacky <thomas.lendacky@amd.com>]

On 6/7/23 11:51, Ard Biesheuvel wrote:
> On Wed, 7 Jun 2023 at 18:08, Tom Lendacky <thomas.lendacky@amd.com> wrote:
>>
>> On 6/7/23 02:23, Ard Biesheuvel wrote:
>>> Before refactoring the EFI stub boot flow to avoid the legacy bare metal
>>> decompressor, duplicate the SNP feature check in the EFI stub before
>>> handing over to the kernel proper.
>>>
>>> The SNP feature check can be performed while running under the EFI boot
>>> services, which means we can fail gracefully and return an error to the
>>> bootloader if the loaded kernel does not implement support for all the
>>> features that the hypervisor enabled.
>>>
>>> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
>>> ---
>>>    arch/x86/boot/compressed/sev.c          | 71 +++++++++++---------
>>>    arch/x86/include/asm/sev.h              |  4 ++
>>>    drivers/firmware/efi/libstub/x86-stub.c | 17 +++++
>>>    3 files changed, 62 insertions(+), 30 deletions(-)
>>>
>>> diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
>>> index 09dc8c187b3cc752..9593bc80c9c6b89d 100644
>>> --- a/arch/x86/boot/compressed/sev.c
>>> +++ b/arch/x86/boot/compressed/sev.c
>>
>> ...
>>
>>> -void sev_enable(struct boot_params *bp)
>>> +u64 sev_get_status(void)
>>>    {
>>>        unsigned int eax, ebx, ecx, edx;
>>>        struct msr m;
>>> +
>>> +     /* Check for the SME/SEV support leaf */
>>> +     eax = 0x80000000;
>>> +     ecx = 0;
>>> +     native_cpuid(&eax, &ebx, &ecx, &edx);
>>> +     if (eax < 0x8000001f)
>>> +             return 0;
>>> +
>>> +     /*
>>> +      * Check for the SME/SEV feature:
>>> +      *   CPUID Fn8000_001F[EAX]
>>> +      *   - Bit 0 - Secure Memory Encryption support
>>> +      *   - Bit 1 - Secure Encrypted Virtualization support
>>> +      *   CPUID Fn8000_001F[EBX]
>>> +      *   - Bits 5:0 - Pagetable bit position used to indicate encryption
>>> +      */
>>> +     eax = 0x8000001f;
>>> +     ecx = 0;
>>> +     native_cpuid(&eax, &ebx, &ecx, &edx);
>>> +     /* Check whether SEV is supported */
>>> +     if (!(eax & BIT(1)))
>>> +             return 0;
>>> +
>>> +     /* Set the SME mask if this is an SEV guest. */
>>> +     sme_me_mask = BIT_ULL(ebx & 0x3f);
>>> +
>>> +     boot_rdmsr(MSR_AMD64_SEV, &m);
>>> +     return m.q;
>>> +}
>>> +
>>> +void sev_enable(struct boot_params *bp)
>>> +{
>>>        bool snp;
>>>
>>>        /*
>>> @@ -410,37 +447,13 @@ void sev_enable(struct boot_params *bp)
>>>         */
>>>        snp = snp_init(bp);
>>>
>>> -     /* Check for the SME/SEV support leaf */
>>> -     eax = 0x80000000;
>>> -     ecx = 0;
>>> -     native_cpuid(&eax, &ebx, &ecx, &edx);
>>> -     if (eax < 0x8000001f)
>>> -             return;
>>> -
>>> -     /*
>>> -      * Check for the SME/SEV feature:
>>> -      *   CPUID Fn8000_001F[EAX]
>>> -      *   - Bit 0 - Secure Memory Encryption support
>>> -      *   - Bit 1 - Secure Encrypted Virtualization support
>>> -      *   CPUID Fn8000_001F[EBX]
>>> -      *   - Bits 5:0 - Pagetable bit position used to indicate encryption
>>> -      */
>>> -     eax = 0x8000001f;
>>> -     ecx = 0;
>>> -     native_cpuid(&eax, &ebx, &ecx, &edx);
>>> -     /* Check whether SEV is supported */
>>> -     if (!(eax & BIT(1))) {
>>> +     sev_status = sev_get_status();
>>> +     if (!(sev_status & MSR_AMD64_SEV_ENABLED)) {
>>>                if (snp)
>>>                        error("SEV-SNP support indicated by CC blob, but not CPUID.");
>>
>> This ends up checking the CPUID path because if SEV isn't advertised in
>> CPUID the returned status value is 0. But it also checks the SEV_STATUS
>> MSR as well. So I think you can remove the SNP / SEV_STATUS check at the
>> end of this function (since that check is identical to this now) and just
>> update the message to indicate not CPUID or SEV status MSR.
>>
> 
> But that one checks for MSR_AMD64_SEV_SNP_ENABLED not
> MSR_AMD64_SEV_ENABLED. Does that matter at all?

Ugh, my bad, I misread that last check. Ignore my comment.

> 
>> The sme_me_mask should probably be cleared at this point before returning,
>> too. Or, alternately, in sev_get_status(), you can update the setting of
>> sme_me_mask to based on MSR_AMD64_SEV_ENABLED being set in the SEV_STATUS MSR.
>>
> 
> I'll go for the latter, seems cleaner not to touch it in that case.

Sounds good.

Thanks
Tom

> 
> --- a/arch/x86/boot/compressed/sev.c
> +++ b/arch/x86/boot/compressed/sev.c
> @@ -422,10 +422,12 @@ u64 sev_get_status(void)
>          if (!(eax & BIT(1)))
>                  return 0;
> 
> -       /* Set the SME mask if this is an SEV guest. */
> -       sme_me_mask = BIT_ULL(ebx & 0x3f);
> -
>          boot_rdmsr(MSR_AMD64_SEV, &m);
> +
> +       /* Set the SME mask if this is an SEV guest. */
> +       if (m.q & MSR_AMD64_SEV_ENABLED)
> +               sme_me_mask = BIT_ULL(ebx & 0x3f);
> +
>          return m.q;
>   }
> 
> 
>>>    /* Search for Confidential Computing blob in the EFI config table. */
>>> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
>>> index 86e1296e87f513b7..081c39b0e8d0d208 100644
>>> --- a/arch/x86/include/asm/sev.h
>>> +++ b/arch/x86/include/asm/sev.h
>>> @@ -207,6 +207,8 @@ bool snp_init(struct boot_params *bp);
>>>    void __init __noreturn snp_abort(void);
>>>    int snp_issue_guest_request(u64 exit_code, struct snp_req_data *input, struct snp_guest_request_ioctl *rio);
>>>    void snp_accept_memory(phys_addr_t start, phys_addr_t end);
>>> +u64 snp_get_unsupported_features(u64 status);
>>> +u64 sev_get_status(void);
>>>    #else
>>>    static inline void sev_es_ist_enter(struct pt_regs *regs) { }
>>>    static inline void sev_es_ist_exit(void) { }
>>> @@ -232,6 +234,8 @@ static inline int snp_issue_guest_request(u64 exit_code, struct snp_req_data *in
>>>    }
>>>
>>>    static inline void snp_accept_memory(phys_addr_t start, phys_addr_t end) { }
>>> +static inline u64 snp_get_unsupported_features(u64 status) { return 0; }
>>> +static inline u64 sev_get_status(void) { return 0; }
>>>    #endif
>>>
>>>    #endif
>>> diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
>>> index abcd5703e9f3f980..1015ef883f5850a4 100644
>>> --- a/drivers/firmware/efi/libstub/x86-stub.c
>>> +++ b/drivers/firmware/efi/libstub/x86-stub.c
>>> @@ -15,6 +15,7 @@
>>>    #include <asm/setup.h>
>>>    #include <asm/desc.h>
>>>    #include <asm/boot.h>
>>> +#include <asm/sev.h>
>>>
>>>    #include "efistub.h"
>>>    #include "x86-stub.h"
>>> @@ -790,6 +791,19 @@ static efi_status_t exit_boot(struct boot_params *boot_params, void *handle)
>>>        return EFI_SUCCESS;
>>>    }
>>>
>>> +static bool have_unsupported_snp_features(void)
>>> +{
>>> +     u64 unsupported;
>>> +
>>> +     unsupported = snp_get_unsupported_features(sev_get_status());
>>
>> This will also set sme_me_mask, but I think that is ok, since on error
>> things will terminate, otherwise sev_enable() should update appropriately
>> later.
>>
> 
> OK
> 
>>> +     if (unsupported) {
>>> +             efi_err("Unsupported SEV-SNP features detected: 0x%llx\n",
>>> +                     unsupported);
>>> +             return true;
>>> +     }
>>> +     return false;
>>> +}
>>> +
>>>    static void __noreturn enter_kernel(unsigned long kernel_addr,
>>>                                    struct boot_params *boot_params)
>>>    {
>>> @@ -820,6 +834,9 @@ void __noreturn efi_stub_entry(efi_handle_t handle,
>>>        if (efi_system_table->hdr.signature != EFI_SYSTEM_TABLE_SIGNATURE)
>>>                efi_exit(handle, EFI_INVALID_PARAMETER);
>>>
>>> +     if (have_unsupported_snp_features())
>>> +             efi_exit(handle, EFI_UNSUPPORTED);
>>> +
>>>        if (IS_ENABLED(CONFIG_EFI_DXE_MEM_ATTRIBUTES)) {
>>>                efi_dxe_table = get_efi_config_table(EFI_DXE_SERVICES_TABLE_GUID);
>>>                if (efi_dxe_table &&